1. Field of the Invention
The subject invention is generally related to blends of polypropylene with liquid crystalline polymers having improved strength and surface characteristics and, more particularly, to compositions containing polypropylene, liquid crystal polymers, and a compatibilizing agent.
2. Description of the Prior Art
Polypropylene is an inexpensive plastic used in a wide variety of applications; however, polypropylene lacks the stiffness and strength required for many applications in which its use would be desirable. Hence, much effort has been put forth in combining polypropylene with other materials to make the polypropylene more suitable for use in such products. It is well known that liquid crystalline polymers can be blended with polypropylene to yield products with improved stiffness. In the past, liquid crystalline polymers have been blended with polypropylene in the same extruder; however, this procedure has proved to be inadequate for many liquid crystalline polymers. Because liquid crystal polymers typically melt at a much higher temperature than polypropylene, the polypropylene is subjected to temperatures in the extruder which are needed to melt the liquid crystal polymer, but which cause polypropylene to degrade. In addition, it is generally desirable to maintain the viscosity of the liquid crystalline polymer below that of the matrix polymer (e.g., polypropylene) in order to form and stabilize certain desirable morphologies. If the temperature in the extruder is maintained to melt the liquid crystalline polymers, then the viscosity of the polypropylene may be lower than that of the liquid crystalline polymer, a characteristic which is not generally desirable. Furthermore, if additional processing of the melt stream is required, as would be the case for film blowing or sheet extrusion, the melt strength of the polypropylene/liquid crystalline polymer blend would be poor if the temperature of the stream is too high (e.g. greater than 280.degree. C.).
European Patent Application 340,655 to Himont Italia shows technology in polymer blending which overcomes problems associated with combining materials that have incompatible thermal processing temperatures. Liquid crystalline polymer fibers are formed in situ in polypropylene by injecting a stream of molten liquid crystal polymer into molten polypropylene at a point which may be near the outlet of the polypropylene extruder (injection could also occur at a point as far back as 15 times the diameter).
A problem with any process in which liquid crystal polymers and polypropylene are blended together is that liquid crystal polymers are generally incompatible with polypropylene. By incompatibility it is meant that because the polymers are chemically quite different, there is a large energy difference at any interface between the polymers. This results in large interfacial surface tension which prevents a drop of one polymer from being finely dispersed in another polymer upon mixing. The large difference in surface energy also inhibits the diffusion of polymer molecules through the interface between two polymers, thereby causing poor dispersion and consequent poor adhesion between the components. In general, manifestations of incompatibility between two polymers in a blend include delamination and separation of the components, poor surface appearance, reduced strength, brittleness, and poor impact properties.
Compatibilizing agents allow one to create polymer blends which possess properties that are different from and are generally not attainable from either of the two components of the blend. Block or graft copolymers possessing segments with chemical structures or solubility parameters which are similar to those of the polymers being blended are effective compatibilizing agents. The compatibilizing agents can be formed separately then added to the polymers that are being compatibilized. Alternatively, the compatibilizing agents can be generated in situ by a reaction between the coreactive functional groups on the polymers. Whether the compatibilizing agent is preformed or generated in situ, the bonding between the polymer chains is generally covalent, ionic or hydrogen. The compatibilizing agent acts as a surfactant in lowering surface tension, promoting finer dispersion and improving interfacial adhesion between the phases. There are a wide variety of known compatibilizing agents and their functions are discussed generally by Gaylord in J. Macromol. Sci.-Chem., A26(8), pp. 1211-1229 (1989).
The maleic anhydride/polypropylene graft polymer, generally known as maleated polypropylene, is a well known compatibilizing agent. As early as 1974, Ide et al. in J. App. Pol. Sci. Vol. 18, pp. 963-974 (1974) disclosed that maleated agent between nylon 6 and polypropylene. With respect to the polypropylene/nylon 6 combination, a graft polymer is formed between polypropylene and nylon 6 by means of a reaction between maleic anhydride substituents of the maleated polypropylene and the amine group of the nylon 6. The polypropylene/nylon 6 blend with the maleated polypropylene had increased tensile strength, elongation at break and impact strength relative to the polypropylene/nylon 6 blend without the maleated polypropylene. Furthermore, a marked improvement in the dispersability of the two components was observed on addition of the maleated polypropylene. As mentioned above, one of the most important reasons to compatibilize polypropylene and nylon 6 in the blend is to improve the impact properties. As the modulus (which reflects stiffness) of the polypropylene is in the range of 1.3 gigaPascals (GPa) and that of the nylon 6 is in the range of 2.0 GPa, the addition of nylon 6 to polypropylene should not affect the stiffness of the polypropylene.
There is a need for an improved polymer alloy which combines the strength and stiffness of liquid crystal polymers with the low cost of polyolefins such as polypropylene and polyethylene. Unlike prior art combinations of liquid crystal polymers and polypropylene which merely have relatively increased stiffness, there is a need for a polymer alloy which also has increased strength. Furthermore, there is a need for a liquid crystalline polymer/polypropylene blend which has better stiffness properties than that which has been achieved by the prior art combinations. In addition, there is a need for a polymer alloy which has an acceptable surface appearance which would allow its use in applications that are subject to close consumer scrutiny such as automobile body parts and the like.